Magnetic Resonance Imaging (MRI) exploits the nuclear magnetic resonance (NMR) phenomena by combining NMR with gradient magnetic fields to allow cross-sectional slice-selective excitation of nuclei within a subject under examination. Typically, a pulse-sequence of radio-frequency magnetic fields (RF pulse) and associated magnetic field gradients is used with further two dimensional (2D) encoding of the NMR signals to create a 2D image of a portion of the subject. A 3D image of the subject can then be obtained by combining many slices together.
By increasing the static magnetic field strength (B0) an improved signal-to-noise ratio may be obtained along with improved spatial resolution in the images created. Ideally, in an MR system the RF pulse should deliver a defined rotation of the nuclear magnetization vector (α°) to provide uniform signal strength over the dimensions of the sample. However, local magnetic and electrical field effects in the subject can lead to spatial inhomogeneity in the local radio-frequency (RF) transverse magnetic field (B1) the nuclei are exposed to. This affects imaging as the MRI pulse sequence employed may not result in the intended rotation of the nuclear magnetisation vector (M). Inhomogeneity in the B1 field is more problematic at B0 fields above 3 T and can lead to imaging artefacts which, in the worst case, are manifested as zero signal in some regions of the image. B1 inhomogeneity effects may also occur at low or medium B0 fields, and when inhomogeneous RF coils such as surface coils are used.
These problems can be addressed by using a multiplicity of transmit coils and activating them in a particular manner to attempt to generate a uniform B1 field. However the manner of activation cannot be pre-computed and must be calculated in real time while the subject is positioned in the imaging device and stationary. Moreover the calculations can take many minutes to complete, and while they are occurring the patient cannot move.
Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.